1. Field of the Invention
The present invention relates to a sealant, an inkjet recording head using a sealant, and a method for manufacturing the same.
2. Description of the Related Art
In an inkjet recording apparatus, recording is performed by ejecting ink from ejection ports toward recording paper. The inkjet recording apparatus generally includes an inkjet recording head that ejects ink from ejection ports and a supply member (supply system) that supplies ink to the recording head. The recording head generally includes an ejection element substrate, a wiring member, and a supporting member.
Examples of a typical recording method using an inkjet recording apparatus include a method using electrothermal transducers. In this method, recording is performed on recording paper as described below. An electrothermal transducer is provided in a pressurizing chamber disposed in the vicinity of an ejection port of an ejection element substrate, and thermal energy is imparted to ink by application of an electrical pulse serving as a driving signal. Using a pressure of ink foaming (boiling) generated due to a phase change of ink at that point, ink is ejected from a minute ejection port. The electrical pulse is applied from the outside using a wiring member (wiring substrate) or the like. The wiring substrate and the ejection element substrate are electrically connected to each other by inner lead bonding (ILB) or the like. Such an electrical connection portion is sealed with a sealant in order to prevent corrosion and short-circuiting of electrodes and wiring, which perform electrical connection, due to ink mist or the like that permeates the space at the time of ejection.
An example of a method for manufacturing such an inkjet recording head will be described below. FIGS. 1A to 1D are views used for explaining the example of a method for manufacturing the recording head. As shown in FIG. 1A, first, a supporting member A (supporting plate) 3a and a supporting member B 3b are joined to each other to form a supporting member (supporting substrate) 3 configured to support ejection element substrates and a wiring substrate. Next, as shown in FIG. 1B, ejection element substrates 2a and 2b are joined to the supporting member 3. Then, as shown in FIG. 1C, a wiring substrate 1 having openings is joined onto the supporting substrate 3 such that the ejection element substrates 2a and 2b are arranged inside the openings. That is, the wiring substrate 1 and the supporting plate 3a are arranged so as to surround the ejection element substrates 2a and 2b. Then, driving electrodes (not shown) of the ejection element substrates and connecting electrodes (not shown) of the wiring substrate are electrically connected with inner leads or the like. More specifically, plating or ball bumps are formed on pads of the ejection element substrates, and inner leads of the wiring substrate are connected to the plating or ball bumps. Finally, as shown in FIG. 1D, an electrical connection portion between each of the ejection element substrates and the wiring substrate 1 is coated and sealed with sealants 4a and 4b. 
Regarding the sealants used in the electrical connection portion between the ejection element substrate and the wiring substrate, it is generally desired that the sealant provided on the upper side (surface side) of the inner leads and the sealant provided on the lower side (supporting member side) of the inner leads have different properties. Specifically, on the lower side of the inner leads, when the sealant is applied from the upper side of the inner leads, the sealant is required to pass through the spaces between the leads and wrap around the lower side of the leads such that the lower side is satisfactorily sealed with the sealant. Therefore, on the lower side of the leads, a sealant having a low viscosity that satisfies the above condition can be used.
On the other hand, in the case where a sealant having such a low viscosity is used on the upper side of the inner leads, the amount of the sealant flowing from the upper side of the leads may be more than necessary, and the upper parts of the leads may not be sufficiently sealed. Therefore, on the upper side of the inner leads, it is required to use a sealant having a high viscosity such that a certain amount or more of the sealant remains on the inner leads and the upper parts of the inner leads can be satisfactorily sealed. Furthermore, on the upper side of the inner leads, when the vicinity of the ejection ports of the recording head is wiped with a blade, the sealant at the electrical connection portion is subjected to friction with the blade. Consequently, the sealant is required to have durability against the friction. Therefore, the sealant is required to exhibit high viscoelasticity after being cured. That is, on the upper side of the leads, it is required to use a sealant that has high viscosity and exhibits a high elastic modulus after being cured.
For the reasons described above, to date, two kinds of sealant have been used. Referring to FIG. 1D, a sealant 4a is used as a sealant used on the upper side of the inner leads, and a sealant 4b is used as a sealant used on the lower side of the inner leads.
As the sealant which can be used on the upper side of the inner leads, which can be cured at a low temperature, and by which print quality and high reliability can be obtained, a photo- and heat-induced, cationic curable resin composition containing an alicyclic epoxy and an oxetane with high cationic reactivity is disclosed in Japanese Patent Laid-Open No. 2007-331334 (Patent Literature 1).
However, when two kinds of sealant having different viscosities are used in sealing the inner leads as those described above, manufacturing steps, such as a sealant application step and the subsequent thermal curing step, tend to become complicated. In particular, since the sealants having different properties are used in succession, conditions for thermal curing become stricter. Furthermore, in the case where the strict conditions cannot be satisfied and the cure state is unstable, two kinds of sealant may dissolve each other, resulting in an insufficient effect.
In order to overcome such a phenomenon, Japanese Patent Laid-Open No. 2010-000700 (Patent Literature 2) discloses a method for manufacturing a recording head including the following step. In this step, an activation energy ray-curable composition is used as a sealant, and during the period from the start to the end of application of the activation energy ray-curable composition to an electrical connection portion, the activation energy ray-curable composition is irradiated with activation energy rays, thereby completing sealing of the electrical connection portion with one kind of sealant.
In this step, since it takes a certain period of time from the irradiation of activation energy rays to the start of curing of the activation energy ray-curable composition as the sealant, the sealant can wrap around the lower side of the leads during that period. As a result, a necessary amount of the sealant can be applied to the lower side of the leads. Then, the activation energy ray-curable composition starts to be cured and is gradually cured. Consequently, the composition does not flow into the lower side of the leads or does not flow out of the sealing points, and the composition is gradually deposited on the upper side of the leads. Thus, it is possible to apply the sealant with a sufficient thickness.
Regarding the resin compositions disclosed in Patent Literatures 1 and 2, sufficient productivity and reliability can be obtained in the current structure of inkjet recording heads. However, hereafter, a reduction in the gap between ink ejection ports and media for the purpose of improving print quality (ink landing accuracy) and use of highly permeable ink (a change in polar solvents, an increase in the amount thereof, or the like) for the purpose of improving image quality are expected, and sealing of electrical connection portions are required to have higher reliability.
Furthermore, in the sealing step using one kind of sealant disclosed in Patent Literature 2, the activation energy ray-curable composition used as the sealant is required to have the following properties. That is, it is required that curing does not proceed immediately after irradiation of the composition with activation energy rays; the composition is gradually cured after the lower side of leads has been buried; and the composition has good electrical properties such that highly reliable electrical connection portions can be formed in an environment in contact with ink after having undergone a heating step. That is, it is required to make a very unique design such that while delaying reactivity (achieving delayed curability), the final cured product has very high electrical properties. There is a small need for such characteristics in the industry, and moreover, there is little knowledge about material design therefor.